Sodium carbonate (Na2CO3), also called soda ash, is an important, high volume chemical produced in the United States and used in the manufacture of glass, chemicals, soaps and detergents and aluminum, as well as in textile processing, petroleum refining and water treatment, among many other uses.
In the United States, almost all sodium carbonate is obtained from subterranean deposits of naturally-occurring trona ore. The largest known trona ore deposits in the United States are located in the Green River basin in southwestern Wyoming, mostly in Sweetwater County, Wyoming, and are typically about 800 to 3000 feet below ground level.
The subterranean deposits of trona ore consist primarily (80-95 wt %) of sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O) and contain lesser amounts of sodium chloride (NaCl), sodium sulfate (Na2SO4), organic matter, and insolubles such as clay and shales.
Trona ore may be recovered from subterranean trona ore deposits, for further processing in surface operations into soda ash or other alkali products, by mechanical mining techniques or by various solution mining methods. The Green River trona ore deposits are presently being commercially mined both by mechanical mining and by solution mining processes.
Mechanical mining, also called dry mining, is carried out underground in the subterranean alkali ore beds by mining crews using complex machinery and includes room-and-pillar and long wall mining methods. Mechanical mining methods are relatively costly due to the upfront cost of sinking mine shafts and continuing need for mining manpower and complex mining machinery. In addition, such mechanical mining methods leave unrecovered a significant fraction of the trona ore in the beds being dry mined, e.g., about 60% unrecovered in room-and-pillar mining and about 30% in longwall mining.
An alternative mining approach developed for recovering minerals from subterranean ore deposits is called solution mining, also sometimes referred to as in situ recovery or in situ leaching. Solution mining can be utilized either as an alternative to or as a supplement to mechanical mining, for the economical recovery of subterranean mineral ore values, such as in the recovery of alkali values from trona ore as soda ash.
In solution mining, the soluble mineral in the underground ore deposit is solubilized with a suitable mining solvent injected via an injection well drilled from the surface down to the underground ore deposit. The resultant mineral-containing mining solution is then withdrawn from the region of the solution-mined ore deposit and pumped to the surface via a withdrawal well for further processing to recover the solubilized mineral values.
Solution mining procedures utilize conventional surface-initiated well drilling technology to drill a borehole from the surface down to the region of the subterranean mineral ore deposit. The drilled well is completed in a conventional manner with casing in the borehole that is sealed in place with cement. Separate wells are normally used for injection of the mining solvent and withdrawal of the mineral-containing mining solution.
Solution mining of trona or other mineral ore deposits is accomplished by injecting water or an aqueous alkaline mining solvent into the ore deposit, via the well. The trona ore deposit may be initially subjected to hydraulic or explosive fracturing to create fissures and openings in the ore to facilitate trona solubilization. The mining solvent is allowed to dissolve or solubilize the mineral ore, with the contact time or residence time being from a few hours to many days. The resulting mining solution (sometimes called mine water or mine liquor) is withdrawn from the region of the ore deposit by pumping the solution to the surface via a withdrawal well. The recovered mining solution is then processed in surface operations to recover the dissolved ore values from the solution, e.g., in the form of soda ash (sodium carbonate) when trona ore is solution mined.
An alkali mining solution from solution mining of a subterranean carbonate mineral ore deposit such as trona typically contains dissolved sodium carbonate and sodium bicarbonate, as well as dissolved organic and inorganic impurities solubilized from the ore deposit. The sodium carbonate values in such alkali solutions are normally recovered as soda ash by various crystallization processes, and the impurities present in the alkali solution are typically removed via a purge stream of crystallizer mother liquor, which is discarded.
Solution mining methods may be employed to recover mineral ore values from virgin (unmined) subterranean ore deposits or may be used for recovering mineral ore values from depleted subterranean ore deposits that have previously been mechanically-worked and abandoned.
Numerous solution mining methods are disclosure in the patent literature for recovery of trona and nahcolite ores, using surface-initiated well drilling techniques to inject a variety of aqueous mining solvents to solubilize the subterranean ore deposit and subsequently recover an alkaline mining solution from the solution-mined ore deposit.
Exemplary solution mining processes for trona are disclosed in U.S. Pat. No. 2,388,009 issued to Pike on Oct. 30, 1945; U.S. Pat. No. 3,050,290 issued to Caldwell et al. (FMC) on Aug. 21, 1962; U.S. Pat. No. 3,119,655 issued to Frint et al. (FMC) on Jan. 28, 1964; U.S. Pat. No. 3,184,287 issued to Gancy (FMC) on May 18, 1965; U.S. Pat. No. 4,264,104 issued to Helvenston et al. (PPG) on Apr. 28, 1981; U.S. Pat. No. 5,043,149 of Frint et al. (FMC) issued Aug. 27, 1991; and U.S. Pat. No. 5,192,164 of Frint et al. (FMC) issued Mar. 9, 1993.
Examples of solution mining procedures applicable to nahcolite ore are described in U.S. Pat. No. 3,779,602 of Beard et al. (Shell Oil) issued Dec. 18, 1973; U.S. Pat. No. 4,815,790 of Rosar et al. (NaTec) issued Mar. 28, 1989; U.S. Pat. No. 6,699,447 of Nielsen et al. (American Soda) issued Mar. 2, 2004; and U.S. Patent Application Publication No. 2009/0200854 A1 of Vinegar (Shell Oil) published Aug. 13, 2009.
An example of a solution mining procedures applicable to salt (sodium chloride) and potash is described in U.S. Pat. No. 2,847,202 issued to Pullen (FMC) on Aug. 12, 1958.
A few solution mining processes describe the use of solution mining techniques to recover alkali values from mined-out sections of trona ore deposits that have earlier been dry-mined, i.e., mechanically-worked.
U.S. Pat. No. 2,625,384 issued to Pike et al. (FMC) on Jan. 13, 1953 describes the solution mining of mined out areas of trona left behind after room-and-pillar dry mining of trona, by introduction of water (the solution mining solvent) and withdrawal of mining solution via underground piping laid in mine passageways in a subterranean dry mining operation. A bulkhead is erected between the solution-mined region and operating dry mining region to prevent the flow of solution mining liquids into the worked section of the dry mining operation.
U.S. Pat. No. 5,690,390 issued to Bithell (FMC) on Nov. 25, 1997 describes a method of solution mining isolated mechanically mined-out areas of soluble trona ore to recover remaining ore reserves, by drilling vertically from the surface then converting the drilling direction to a substantially horizontal well bore at a predetermined distance below the ground level. The horizontal drilling is directionally drilled parallel to and within the trona ore body to form a well bore that connects to the mined-out area. Additional separate wells originating from the surface are drilled for injection of a mining solvent into the mined out cavity, for connecting the horizontal well bore to an operational mine area, and for pumping recovered solution mining liquor to the surface, all as shown in FIG. 1 of Bithell '390.
Surface processing operations for recovering soda ash from dry-mined trona ore and from alkali mining solutions obtained from trona solution mining are described in U.S. Pat. No. 5,262,134 of Frint et al. (FMC) issued Nov. 16, 1993. The Frint et al. '134 patent describes the recovery of sodium carbonate values from mining liquor obtained from solution mining of subterranean trona ore deposits, via sequential crystallizations of sodium sesquicarbonate and sodium carbonate decahydrate, the latter then being recrystallized as sodium carbonate monohydrate. The Frint '134 patent contains descriptions of various prior art trona ore solution mining techniques and of the “sesquicarbonate” and “monohydrate” soda ash recovery processes applicable to dry-mined trona ore, and those disclosures of U.S. Pat. No. 5,262,134 are hereby incorporated by reference into the present specification.
The present invention provides a method of solution mining subterranean mineral ore beds without the need to drill costly injection or withdrawal wells from the surface. The method of this invention utilizes an existing mechanically-worked subterranean mining operation as the operational base for effecting an underground solution mining operation. In addition, a remote region to be solution-mined is connected via a drilled connective borehole to the operating mechanically-worked mine operation, utilizing safeguards against inadvertent entry of solution mining liquids into the operating mechanically-worked mine.